A construction machine such as a wheel loader does not frequently drive continuously at a constant speed. Therefore, the construction machine requires driving capability different from that of general vehicles such as automobiles.
FIGS. 50A to 50F are plan views showing a V-patterned operation which occupies a large part of the operation of the wheel loader. As shown in FIG. 50A, a wheel loader 100 is driving forward toward dirt (earth) 102 to shovel the dirt 102. Then, as shown in FIG. 50B, the wheel loader 100 is shoveling the dirt 102. In this case, the wheel loader needs a great traction force in a range from zero speed to a very low speed (stall mode).
After completing shoveling the dirt 102, the wheel loader 100 is driving backward and then forward toward a truck 101 as shown in FIG. 50C (switch back). Then, as shown in FIG. 50D, the wheel loader 100 is driving forward toward the truck 101. Then, as shown in FIG. 50E, the wheel loader 100 is loading the dirt 102 into the truck 101.
After completing loading the dirt 102 into the truck 101, the wheel loader 100 is driving backward and then forward toward the dirt 102 again (switch back) as shown in FIG. 50F.
In the above mentioned V-patterned operation, a great traction force is needed in a very low-speed range, or acceleration and deceleration must be repeated and the switch back must be repeated in a low-speed range. Such a need is unique to the construction machine.
In recent years, attention has been focused on hybrid vehicles, which include a combination of an engine and an electric motor/generator, and a variety of associated devices have been developed. By way of example, Japanese Patent No. 3344848 discloses a starting device for hybrid vehicles (hereinafter referred to as Prior Art Example 1) and Japanese Laid-Open Patent Application Publication No. Hei. 9-14385 discloses a driving device for hybrid vehicles (hereinafter referred to as Prior Art Example 2).
In the Prior Art Example 1, a target rotational speed of gears coupled to an output shaft of an engine is determined based on the extent to which the accelerator is actuated, and an electric rotation device is driven to generate a braking torque, thereby controlling the electric motor/generator so that the rotational speed of the gears becomes the set target rotational speed. With such a configuration, the vehicle can be started efficiently.
In the Prior Art Example 2, ring gears of an epicyclic gearing are coupled to an electric motor/generator, a sun gear of the epicyclic gearing is coupled to the engine, and drive power is output from a carrier of the epicyclic gearing to drive wheels, while two of the ring gears, the carrier, and the sun gear are integrally coupled at the point in time when their coupling rotational speeds become equal, and are integrally rotated at the coupling rotational speed or higher. With such a configuration, the vehicle is able to start smoothly.
To obtain the above mentioned driving capability which is unique to the construction machine, wheel loaders of medium and larger sizes are typically equipped with torque converters. FIG. 51 is a graph showing the relationship between torque ratio and efficiency, and torque ratio and absorption torque coefficient, with respect to speed ratio of the torque converter. FIG. 52 is a graph showing the relationship between input torque of the torque converter and engine speed.
Turning to FIG. 51, it is confirmed that the torque converter has a low efficiency as a whole and a very low efficiency particularly in a low-speed ratio (see R1 region). As described above, the wheel loader requires driving capability in the low-speed range, such as acceleration, deceleration, stall, or switch back, and drives more frequently in the low efficiency region of the torque converter. Therefore, efficiency of the torque converter in the low-speed range must be improved.
Turning to FIG. 52, a transmitted torque depends on the rotational speed of the input shaft (output shaft of the engine). Therefore, it is confirmed that the transmitted torque is smaller when the engine speed is lower (see R2 region). For this reason, during the acceleration, a sufficient torque cannot be transmitted before the engine speed rises. This is disadvantageous to the construction machine which repeats acceleration and deceleration within a short distance.
In the construction machine such as the wheel loader which is configured as a hybrid vehicle, to obtain driving capability which is equal to or higher than that obtained using the torque converter, an engine torque, a motor/generator torque, and upshifting or downshifting of a gear position must be controlled mainly in a state where a direct-coupling clutch is in an off-state in a low-speed range.
Whereas in the Prior Art Example 1 and the Prior Art Example 2, control is executed during a period from when the vehicle is in a stopped state until the direct-coupling clutch is turned on or brought into engagement, the control for the engine torque, the motor/generator torque, and the upshifting or downshifting of the gear position is not executed in a state where the direct-coupling clutch is not turned on or in a disengagement state. For this reason, it is difficult to attain a hybrid construction machine using the Prior Art Examples 1 and 2.